Adjuvant systemic therapy, including chemotherapy, hormonal therapy, and trastuzumab, markedly reduces the risk of metastatic recurrence and death for selected patients with breast cancer. Guideline-concordant use of chemotherapy and optimal dose intensity are critical factors in achieving maximal benefit of treatment. Similarly, guideline-concordant prescribing and optimal patient adherence to hormonal therapy are critical factors in maximizing benefit. Variations in the quality of adjuvant therapy may account for racial, ethnic, and social disparities in disease-free and overall breast cancer survival. The underlying reasons for observed patterns and correlates of quality adjuvant chemotherapy and hormonal therapy are unknown. The purpose of this population-based study is to investigate disparities in the quality of adjuvant systemic therapy according to patient demographic and social support factors. In addition, we will investigate how the accuracy of measurement of key pathologic factors-specifically estrogen receptor (ER) status and human epidermal growth factor receptor-2 (HER2) status-may affect decision-making and thus disparities in the quality of systemic therapy. We propose to investigate these factors through medical record review, patient survey, and repeat assessment of ER and HER2 status on primary tumor specimens. We have already assembled one of the largest and most ethnically diverse population-based samples of patients newly diagnosed with breast cancer in the metropolitan areas of Detroit and Los Angeles. The proposed study will expand the scope of research using this population to address the quality of clinical evaluation and disparities in the quality of systemic treatment for women with breast cancer. The Specific Aims of this study are (1) to examine patterns and correlates of quality of adjuvant chemotherapy in a population-based sample of women, (2) to examine patterns and correlates of quality of breast cancer hormonal therapy in a population-based sample of women with breast cancer, and (3) to estimate the frequency of classification error in key pathologic variables-ER and HER2 status-in a population-based sample of women with breast cancer and explore the impact of such error on receipt of optimal adjuvant systemic therapy. We propose to investigate these factors through patient interview, medical record review, and repeat assessment of pathologic variables on primary tumor specimens. The results of this study will be used to advance methods in oncology outcomes research and to inform policy and practice interventions to improve the quality of breast cancer care in the United States. Public Health Relevance: The results of this study will be used to inform policy and practice interventions to address disparities in the quality of breast cancer care.